"A still tongue makes a wise head" is an idiom. Option A is correct.
This saying may mean that someone who does not talk much, but listens to other people, probably has experience, knowledge and good judgement.
An idiom is a type figure of speech which should not be taken literally. It is established by usage that has a meaning not necessarily deductible from those of the individual words.
Idioms convey a particular sentiment, but they do not literally mean what the individual words themselves mean.
